Groundwater exploration with AEM in the Boteti Area, Botswana

Abstract

As part of a project by the Department of Water Affairs, Botswana to evaluate groundwater resources in the Boteti area in Botswana airborne EM data were acquired with the TEMPEST system. The project area is characterised by a palaeolake system bordered by elevated terrain to the south. Lower-lying areas are covered by a thick layer of Kalahari Beds with saline groundwater generally situated less than 20 m below surface. The elevated areas have Karoo sediments, constisting of sand- and mudstones, at shallow depths. For the palaeolake terrain the target aquifers are fresh water zones associated with recharge pans above the saline water table. The correlation of AEM-derived conductivity-depth profiles with drillhole records show that the conductivity of Kalahari Beds is primarily a function of clay content, water saturation and water salinity. The shallow conductivity structure outlines several resistive zones located within Boteti River alluvium beneath the present and past river channels. These prospective zones are interpreted to have a low clay content and to be saturated with fresh water. In the elevated terrain the target aquifers are sandstones sandwiched in-between dry alluvium and mudstone. The comparison of AEM results with drillhole data indicates that sand- and mudstones have low and high conductivities, respectively. Favourable locations for freshwater exploitation include palaeochannels incised into the mudstone and shallow fractured sandstone units.